Method and apparatus for enhancing and maintaining character quality in thermal printers

Abstract

The quality of thermally printed characters is enhanced by controlling the time at which and the time for which power is applied to the resistive printing elements in a battery-operated moving-head thermal dot matrix printer. By sequentially strobing the elements in the pattern of the character to be formed as the print head moves across thermal sensitive paper, a high-quality slanted character is printed and parasitic losses are reduced. By inversely varying the time power is supplied to each dot as battery voltage varies, character quality is maintained and useful battery life is extended.

I claim: 
     
       1. A printer for printing characters on a printing medium comprising: a printer head, having a plurality of spaced transducers mounted in a line thereon, movably mounted in close proximity to the printing medium, the line of transducers being oriented transverse to the direction of head movement;   motive means coupled to the printer head for driving the printer head past the printing medium at a predetermined rate;   timing means for producing timing signals;   character generating means coupled to the timing means for generating character data signals in response to timing signals therefrom;   slant generating means coupled to the timing means for generating periodic, sequentially-timed command signals at a preselected repetition rate in response to timing signals received therefrom; and   gating means coupled to the printer head, the slant generating and the character generating means for selectively activating successive ones of the transducers in response to command and character data signals to print characters on the printing medium as a matrix of rows and columns of dots, the interval between rows being determined by the spacing between transducers and the interval between columns being determined by the repetition rate of the command signals and the rate at which the printer head is driven.   
     
     
       2. A printer as in claim 1 wherein: the printing medium produces a mark on the surface thereof in response to heat generated in close proximity thereto; and   the transducers generate heat in response to an electrical signal applied thereto.   
     
     
       3. A printer as in claim 1 wherein the slant generating means includes a circulating shift register having a plurality of output ports for coupling command signals therefrom and first gating means coupled to the shift register for controlling the contents thereof. 
     
     
       4. A printer as in claim 1 powered by a battery wherein the slant generating means includes duty cycle generating means for controlling the period of the command signals generated thereby to activate the transducers to produce printed characters having uniform quality on the printing medium. 
     
     
       5. A printer as in claim 4 wherein: the slant generating means includes a circulating shift register having a plurality of output ports; and   the duty cycle generating means includes a capacitor and a capacitor charging circuit coupled to the battery for charging the capacitor to a voltage substantially equal to the voltage thereof, and having an output port for coupling an electrical signal representing the capacitor voltage therefrom; a comparator having an output port and at least two input ports, one of the input ports being coupled to a reference voltage representing the voltage required for the transducers to produce printed characters having uniform quality on the printing medium and the other of the input ports being coupled to the output port of the capacitor charging circuit, said comparator being effective for comparing the voltages applied to the input ports thereof and for providing a signal at the output port representing the time it takes the capacitor to charge to a voltage substantially equal to the reference voltage; and second gating means coupled to the comparator and to the shift register for controlling the contents thereof in response to the signal at the output port of the comparator, said contents being effective for controlling the period of the command signals.   
     
     
       6. A printer as in claim 5 wherein the time it takes the capacitor to charge to a voltage substantially equal to the reference voltage is approximately equal to the length of time the transducers must be activated at the battery voltage available to produce printed characters having uniform quality on the printing medium. 
     
     
       7. A printer as in claim 4 wherein the duty cycle generating means controls the period of the command signals inversely as a function of the battery voltage. 
     
     
       8. A printer as in claim 7 wherein the function of the battery voltage is substantially exponential. 
     
     
       9. A printer as in claim 4 wherein the command signals include an on-time and an off-time, said on-time being set by the duty cycle generating means as a function of the magnitude of the battery voltage and being effective for enabling activation of the transducers. 
     
     
       10. A printer as in claim 9 wherein the on-time of the command signals is an inverse function of the magnitude of the battery voltage. 
     
     
       11. A printer as in claim 4 wherein: the slant generating means includes a circulating shift register having a plurality of output ports;   the duty cycle generator includes: sampling means for determining the magnitude of the battery voltage;   comparator means coupled to the sampling means, for comparing the magnitude of the battery voltage with a reference voltage to determine the length of time the transducers must be activated at the magnitude of battery voltage available to produce printed characters having uniform quality on the printing medium; and   gating means, coupled to the comparator means, for controlling the contents of the circulating shift register, said contents being effective for controlling the period of the command signals.     
     
     
       12. A printer as in claim 1 for printing slanted characters wherein the slant of the columns of the printed characters is determined by the sequential timing of the command signals and the rate at which the printer head is driven. 
     
     
       13. A method for printing characters on a printing medium comprising the steps of: supplying power;   driving a printer head having a plurality of spaced transducers mounted in a line aligned thereon transverse to the direction of head movement past the printing medium in close proximity thereto and at a predetermined rate;   producing timing signals;   generating character data signals in response to the timing signals;   generating periodic, sequentially-timed command signals at a preselected repetition rate in response to the timing signals; and   activating successive ones of the transducers selectively in response to command and character data signals to print characters on the printing medium as a matrix of rows and columns of dots, the interval between rows being determined by the spacing between transducers and the interval between columns being determined by the repetition rate of the command signals and the rate at which the printer head is driven.   
     
     
       14. A method as in claim 13 wherein: the printing medium produces a mark on the surface thereof in response to heat generated in close proximity thereto; and   the transducers generate heat in response to an electrical signal applied thereto.   
     
     
       15. A method as in claim 13 wherein the step of generating command signals includes coupling command signals from a circulating shift register having a plurality of output ports and controlling the contents thereof. 
     
     
       16. A method as in claim 13 wherein: the step of supplying power comprises the step of supplying power from a battery; and   the step of generating periodic, sequentially-timed command signals includes the step of controlling the period of the command signals to activate the transducers to produce printed characters having uniform quality on the printing medium.   
     
     
       17. A method as in claim 16 wherein: the step of controlling the period of the command signals includes the step of controlling the contents of a circulating shift register having a plurality of output ports which comprises the steps of: a. charging a capacitor to a voltage substantially equal to the voltage of the battery;   b. comparing the capacitor voltage with a reference voltage representing the voltage required for the transducers to produce printed characters having uniform quality on the printing medium;   c. providing a signal representing the time it takes the capacitor to charge to a voltage substantially equal to the reference voltage; and   d. controlling the contents of the circulating shift register in response to the signal representing the time it takes the capacitor to charge to a voltage substantially equal to the reference voltage, said contents being effective for controlling the period of the command signals.     
     
     
       18. A method as in claim 16 wherein the step of controlling the period of the command signals includes controlling that period inversely as a function of the battery voltage. 
     
     
       19. A method as in claim 16 wherein the function of the battery voltage is substantially exponential. 
     
     
       20. A method as in claim 16 wherein: the step of generating command signals includes coupling command signals from a circulating shift register having a plurality of output ports; and   the step of controlling the period of the command signals includes the steps of: determining the magnitude of the battery voltage;   comparing the magnitude of the battery voltage with a reference voltage to determine the length of time the transducers must be activated at the magnitude of battery voltage available to produce printed characters having uniform quality on the printing medium; and   controlling the contents of the circulating shift register.     
     
     
       21. A method as in claim 13 for printing slanted characters wherein the slant of the columns of the printed characters is determined by the sequential timing of the command signals and the rate at which the printer head is driven.